418 t»E. ll. BEOOM ON IHE 



into the organ from the outer side. It also occurs in Echidna 

 but no higher mammal retains it. The appearance of the 

 cartilages in the Marsupials and Edentata, however, suggests 

 the belief that the higher forms are descended from animals 

 which had a turbinal in the organ. The relations of the 

 posterior part of the organ to the septum in SpJienodon are 

 typically mammalian. 



With regard to the structure of the organ, so far as can be 

 made out from the sections of the embryos I have examined, 

 the affinities are much more with the Squamata than with the 

 Mammalia. The lower wall of the organ is composed of two or 

 three layers of rounded epithelial cells, apparently without 

 cilia: the upper and inner wall is formed of closely-packed 

 bunches of neuro-epithelium, as in the Squamata. In Mammals 

 the sensory epithelium is usually confined to the inner and lower 

 wall of the organ, but in the Monotremes it covers also the 

 upper wall of the organ. 



The study of the organ of Jacobson and its relations helps 

 considerably towards the solution of the problem of the affinities 

 of SjyJienodon. By Huxley, Osawa, and others, Sphenodon has 

 been held to be a true lizard. By Gadow and many others it 

 has been looked upon as an extremely jn'imitive reptile not very 

 far removed from the Batrachians, and belonging to an order 

 from which almost all other reptiles have descended. It will 

 probably only be possible to definitely settle the position of 

 Sphenodon when palaeontology has advanced much further than 

 at present, but it seems to me that enough can be said on each 

 side of the question to demand for the opposing views most 

 careful consideration. In many respects Sphenodon bears a 

 closer resemblance to the lizards than to other reptiles, but the 

 characters which they have in common are probably primitive 

 and shared by most of the early lizard-like forms. It seems 

 impossible that lizards with a distinct prosquamosal can have 

 descended from a 8phenodon-\\k.Q form in which that bone is 

 lost ; but lizards and Sphenodon ma}^ both have had a common 

 ancestor in Permian times which had a distinct prosquamosal 

 and a fully-roofed temporal regiou. Such an ancestor would 

 probably not be far removed from the Cotylosauriaus, one of 

 which was the remote mammalian ancestor. Sphenodon, though 

 it has advanced far from the Cotylosaurian state, still retains a 

 number of the primitive characters, and the organ of Jacobson 



